1. Field of the Invention
The invention relates to a process for isolating hydroxypivalic acid from aqueous solution by removal of the water by azeotropic distillation under reduced pressure using an organic solvent or solvent mixture suitable for this, and subsequent crystallization of the hydroxypivalic acid from an organic solvent mixture which is composed of a polar and a non-polar component, one or both components of which have been employed in the preceding azeotropic distillation.
2. Description of the Related Art
In addition to potassium permanganate oxidation of neopentylglycol and Cannizzarro reaction of hydroxypivalaldehyde, both of which are relatively unattractive from the industrial aspect, hydroxypivalic acid can be prepared by air/O.sub.2 oxidation of neopentylglycol or by H.sub.2 O.sub.2 oxidation of hydroxypivalaldehyde. The oxidation of neopentylglycol with O.sub.2 or air over a Pd/C or Pt/C catalyst in alkaline aqueous solution to give hydroxypivalic acid is described, for example, in JP 53/77 010 or in U.S. Pat. No. 3,799,977. In both patents, no instructions are given for liberation of the hydroxypivalic acid obtained as the Na salt. Furthermore, the yield and selectivity data are lacking completely in U.S. Pat. No. 3,799,977, and a very dubious yield of 100% based merely on the comparison of IR bands of the sodium hydroxypivalate prepared according to JP 53/77 010 with a pure sodium hydroxypivalate sample is stated in JP 53/77 010. As an alternative to this process, hydroxypivalic acid can also be synthesized by oxidation of hydroxypivalaldehyde with hydrogen peroxide. This process was described for the first time in Monatshefte fur Chemie 95, (1964), 410 and taken up again in an improved variant in 1968 in JP 43/24 888. After the hydrogen peroxide oxidation, the hydroxypivalic acid is obtained in aqueous solution. For isolation, according to Monatshefte, the hydroxypivalic acid is converted into the Na salt by addition of Na.sub.2 CO.sub.3. This salt is precipitated by addition of 10 times the volume of acetone and is filtered off. The residue is dissolved in a little water and the hydroxypivalic acid is liberated with the stoichiometric amount of H.sub.2 SO.sub.4 and taken up in CHCl.sub.3. The water-moist CHCl.sub.3 solution is dried with Na.sub.2 SO.sub.4, the Na.sub.2 SO.sub.4 is filtered off and the CHCl.sub.3 is evaporated off. A crude hydroxypivalic acid having a melting range of 100.degree. to 122.degree. C. (literature: 124.degree.-126.degree. C.) remains. This wide melting range indicates that the hydroxypivalic acid contains a considerable amount of entrained impurities. Assuming the crude hydroxypivalic acid according to the Monatshefte is pure, the hypothetical yield is about 69% of the theoretical yield, based on the hydroxypivalaldehyde employed. The authors obtain truly pure hydroxypivalic acid having a melting point of 125.degree. to 126.degree. C. only after further crystallizations, first from CHCl.sub.3 and then from water. Since hydroxypivalic acid has an excellent water-solubility, it must be assumed that the yield of pure hydroxypivalic acid after these further recrystallizations falls significantly below 50% of the theoretical yield. The increase in volume due to the addition of acetone and the large number of working-up steps also make this process unattractive industrially.
JP 43/24 888 now describes a variant of the above process using a catalyst which is capable of dissociating hydrogen peroxide. Very fine particles of gold, platinum, silver or glass and also UV light of wavelength 2000 to 4000 .ANG. are mentioned as such catalysts. The difficulties of obtaining hydroxypivalic acid by crystallization from aqueous solution are dealt with further in this patent application; no material which has a melting point above 120.degree. C. was to be isolated from water. The authors therefore remove the water completely by azeotropic distillation with a suitable solvent, such as benzene, toluene, cyclohexane, n-butanol and the like; however, only benzene is employed in the patent examples. After the dehydration, the solvent is stripped off and the crude hydroxypivalic acid is subjected to fractional distillation under reduced pressure. During reworking of Example 2 from JP 43/24 888 with toluene instead of benzene (for industrial hygiene reasons), the water was distilled off azeotropically. In the subsequent distillation of hydroxypivalic acid, only 0.64 mol of the 2.35 mol of hydroxypivalic acid originally present in the solution could be distilled off under 20 mbar. Bifunctional hydroxypivalic acid forms polyesters and anhydrides during distillation, which remain as a dark brown distillation residue. The distillate must furthermore be kept in the melt at approx. 125.degree. C. This can also lead to losses in yield and quality due to oligo-/polymerization reactions. The hydroxypivalic acid distillate must then still be cooled on a belt and comminuted mechanically.
Alternatively, after the azeotropic dehydration, the hydroxypivalic acid is crystallized out of the entraining agent benzene according to JP 43/24 888 by cooling, but must be recrystallized from benzene again in order to achieve the required quality.
Since benzene is classified as carcinogenic, crystallization from this solvent is now regarded as unacceptable from the industrial hygiene aspect. Our own works have shown that after azeotropic dehydration of an oxidation batch with the less unacceptable toluene, subsequent crystallization from the solvent led to a tacky hydroxypivalic acid having a content of less than 95% (GC); this is evidently to be attributed to the abovementioned entrained impurities.
The processes described to date for isolation of hydroxypivalic acid are not very suitable for industrial preparation of hydroxypivalic acid because of the number of working-up steps and the marked tendency to form polyesters/anhydride at higher temperatures.